It is well known in the prior art of electrically driven vehicles, such as mine cars, to have an external power supply connected to the vehicle by means of a cable which is wound on a reel mounted on the vehicle. The connection to the main power supply is referred to as the tie-point. The tie-point is normally located somewhere near the midpoint of expected vehicle travel so as to reduce by half the needed length of cable as compared to when the tie-point is located at one end of the expected path of travel. As the vehicle approaches the tie-point the cable reel need be driven in a direction which takes up the cable slack and winds the cable on a reel, this being referred to as the pay-in condition. When the vehicle is moving away from the tie-point, the tension on the cable unwraps the cable from the reel proportional to the distance of vehicle travel, this being referred to as the pay-out condition. During pay-out, if the reel is allowed to free-wheel, inconsistent velocities of the vehicle induce momentary excessive turning of the reel due to reel inertia, causing cable slack, or instantaneous extra tension on the cable, both of which are undesirable. Furthermore as the vehicle passes under the tie-point, excessive deceleration and acceleration forces are applied to the cable and cable reel which tends to cause cable breakage.
One prior art system is taught in Maier U.S. Pat. No. 4,114,827 issued Sept. 19, 1978 wherein the cable reel drive consists of an electric reel motor driving a fixed displacement pump which in turn drives a fixed displacement motor connected to the cable reel. Interposed between the pump and the motor is a valve which automatically by-passes pump flow when the cable reel is paying out. Another prior art construction is Boettchler et al U.S. Pat. No. 3,788,575 issued Jan. 29, 1974 wherein the control of the hydraulic motor driving the cable reel is obtained by sensing cable tension directly to operate a flow control valve. This system controls motor speed and is not a pressure controlled system. U.S. Pat. Nos. 3,749,988 and 3,749,989 issued July 31, 1973 to John Pittner detect coil diameter and the coil rpm to modulate a D.C. electric motor field in a reel drive. None of these systems modulate the torque of the cable reel drive relative to a friction reversal between a paying in or a paying out condition, or generate a transient condition signal proportional to reel acceleration to further modify reel drive.
U.S. Pat. No. 4,108,264, issued Aug. 22, 1978, to Tanaka teaches a cable reel mechanism wherein slack angle is measured to modify the electric drive on the cable wheel and provide braking against rewind. While the patent recognizes the adverse disadvantages of vehicle velocity changes, such changes are compensated for by means of gear shift lever switches. There is also no recognition of cable reel acceleration changes as the vehicle passes a tie-point.